linux-kernel - Re: [PATCH v2 0/5] iommu/vt-d: Ensure memory ordering in context & root entry updates

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Message-ID: <20260105191410.GJ125261@ziepe.ca>
Date: Mon, 5 Jan 2026 15:14:10 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Dmytro Maluka <dmaluka@...omium.org>
Cc: David Woodhouse <dwmw2@...radead.org>,
	Lu Baolu <baolu.lu@...ux.intel.com>, iommu@...ts.linux.dev,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>, linux-kernel@...r.kernel.org,
	"Vineeth Pillai (Google)" <vineeth@...byteword.org>,
	Aashish Sharma <aashish@...hishsharma.net>,
	Grzegorz Jaszczyk <jaszczyk@...omium.org>,
	Chuanxiao Dong <chuanxiao.dong@...el.com>,
	Kevin Tian <kevin.tian@...el.com>
Subject: Re: [PATCH v2 0/5] iommu/vt-d: Ensure memory ordering in context &
 root entry updates

On Mon, Jan 05, 2026 at 07:54:53PM +0100, Dmytro Maluka wrote:
> > Like AMD and ARM build the new PASID entry on the stack and then it
> > should be copied to the DMA'able memory in a way that is consistent
> > with the HW's atomicity granual, paying attention not to 'tear' it.
> 
> As I understand, the "consistent with the HW's atomicity granual, paying
> attention not to 'tear' it" part is already fulfilled for PASID entries
> (and with this series, for context entries as well):
> 
> static inline void pasid_set_bits(u64 *ptr, u64 mask, u64 bits)
> {
> 	u64 old;
> 
> 	old = READ_ONCE(*ptr);
> 	WRITE_ONCE(*ptr, (old & ~mask) | bits);
> }
> 
> I've been assuming it's ok to manipulate other bits in place as long as
> we take care to only do that while the present bit it cleared (i.e.
> while the entry is ignored by hardware)?

If these are only done while non-present then the only issue is
missing a barrier before setting present, that should be a one line
patch, no?

> So IIUC the only problem with this approach is the redundancy: we do
> this READ_ONCE+WRITE_ONCE for each invididual field in a PASID entry.

You don't need READ_ONCE if there isn't another thread concurrently
writing, and WRITE_ONCE is pointless if the HW is promising not to
read it due to non-present.

> So while I agree it would be more more natural to build whole entries,
> and the existing way looks strange and not the most efficient, I'm
> wondering if it is causing any actual correctness issues (apart from
> those addressed by this series).

It prevents doing the replace operation, which is a correctness issue
for VMs.

Jason